It is anticipated that the transport capacity of next-generation optical access/metro networks (<600 km) will migrate to 40-Gb/s or 100-Gb/s per channel in the near future due—in part—to the proliferation of video traffic. However, unlike high-speed long-haul systems (1000+ km) that can offset high technological implementation costs, it is essential that technologies employed in next-generation optical access/metro networks be both highly flexible and cost-efficient.
As a result of its high spectral efficiency, resilience to linear dispersion, and efficient digital signal processing (DSP)-based implementation, optical orthogonal frequency division multiplexing (OFDM) has emerged as an attractive candidate for next-generation fiber-optic systems. Moreover, optical OFDM-based Multiple Access (OFDMA) is particularly attractive for next-generation optical access/metro systems due to its application transparency and bandwidth flexibility.
Polarization-multiplexed (POLMUX) OFDM transmission with direct (non-coherent) detection has been shown to further increase spectral efficiency at ultra high-speeds while advantageously requiring a reduced optical receiver complexity and cost. By achieving record 40+ Gb/s data rates with simplified optical receivers, POLMUX-OFDMA with direct detection (DD) is a highly-promising technology for future fiber-based access/metro systems.
However, while POLMUX-OFDM-DD systems reduce receiver complexity compared to coherent receivers, they also increase the complexity of the required post-photodetection electronic digital signal processing (DSP). More particularly—and due to direct detection—cross-polarization interference will occur in the optical receiver, which must be corrected, or equalized, in post-photodetection DSP. Operationally, this DSP-based equalization requires both the computation of a 4×4 matrix inverse and the multiplication of incoming data with the inverted matrix (i.e. data equalization). Consequently, both of these steps can prohibitively increase DSP receiver complexity and, if performed in a sub-optimal way, can also enhance noise effects.
Previously, we have proposed receiver-end processing algorithms that can notably reduce the complexity of the 4×4 matrix inverse computation. However, the computational complexity of the equalization step remained unchanged. Since the equalization step must be performed significantly more often than the required matrix inversion, it subsequently became a limiting factor in overall computational complexity. Moreover, in our previous work, the equalization step was performed sub-optimally with respect to theoretically-optimal maximum likelihood (ML) equalization, which can enhance noise effects and degrade the bit error rate (BER) of the system. Consequently, a processing algorithm that enables computationally-efficient ML equalization and reduces overall complexity would represent a significant advancement in the art as it pertains to high-speed, real-time POLMUX-OFDM-DD systems.